Research and development efforts in the photographic materials and paper materials industries often focus on detecting various types of imperfections in a moving coated web, e.g. a sensitized film or paper web. These imperfections may, for example, result from disturbances in process of coating the film base or paper with sensitized layers. Research and development efforts attempt to isolate, through process modeling, the source of an on-going disturbance-type in a coating process once the imperfections are identified. In addition, in the commercial production of such sensitized media, the detection and location of imperfections in wide and long webs is conducted so that the imperfections may be avoided when the webs are severed into film and paper strips for packaging in light tight containers. Moreover, on-line web imaging may be used for process control applications.
Coating imperfections of particular interest are continuous-type imperfections or streaks and point-type or two-dimensional imperfections. These imperfection types, which can occur in one or more coating layers on a support web, are typically indicative of a disturbance or design related problem in the coating process.
An effective on-line imperfection recognition system and method would enable one to discern, characterize and confirm various models of the coating process, thereby determining the disturbance causing such an imperfection. Two significant issues, however, must be addressed by any imperfection recognition system before adequate optical data can be collected from sensitized coatings under examination. First, the system must be able to extract small density changes from the obtainable spatial and temporal noise background. Secondly, the system must provide adequate, uniform illumination within the spectral bandwidth of the usable contrast range, while avoiding fogging of any sensitized coating layers.
State-of-the-art efforts to quantify moving web disturbances have most commonly been implemented as laser scanning systems. For example, continuous laser beams are often swept by multifaceted polygon mirror scanners across moving webs of sensitized film or paper, and focused with dedicated optics onto a discrete detector such as a photomultiplier tube. Various detector configurations enable data acquisition in either a reflective or transmissive mode. Unfortunately, such laser scanner packages can be expensive and typically have limited anomaly detection capabilities.
Specifically, such laser scanning packages are almost universally unable to process data associated with very narrow lines and streaks which may be embedded in noise. (Also, laser scan output processing packages, in general, remain less sophisticated than those accompanying state-of-the-art imaging technologies such as solid state cameras.) Therefore, a need is recognized to exist today in the photographic and paper materials industry for a more effective and less expensive technique to extract and characterize imperfections from background data including inherent noise variations, and particularly low-level, narrow continuous-type imperfections or streaks in a moving coated web.
A potentially effective means of web coating streak detection may be realized with solid state image sensing technology. Solid state CCD cameras have been described in the above-referenced '318 application in the detection and analysis of a number of coating imperfections. A two- dimensional 512.times.512 pixel array CCD camera is described for detecting light transmitted through the moving web that is modulated in intensity by the coating layers. Infrared light transmitted through the web is generated by a light integrating sphere that illuminates the array area. Both continuous and strobed illumination may be employed, depending on the mode of detection of streaks or point-type imperfections.
Linear light integrators are well known light sources in the art of image scanning and digitizing, where an image is fixed on a media, e.g. paper sheets, movie film, radiographic film sheets or the like. Linear light integrating cavities are typically used in scanning image frames of developed negative or positive film strips in film image frame digitizing systems of the type described in commonly assigned U.S. Pat. No. 4,868,383 and in the above referenced '639 application. Such linear light sources typically employ an external lamp(s) and lens and filter system which direct a light beam into the integrating cavity through an input port(s). As identified in the above-referenced '383 patent, the lamp(s) can be, for example, xenon or tungsten halogen lamp(s). As described therein, filtration for blocking infra-red wavelengths and emphasizing the blue light to scan negative film is necessary with these lamps.
In the above referenced '639 application, an improved linear light integrator is described employing a light pipe with spectral filtration for filtering the line of light emitted from the integrating cavity. With such filtration of the emitted light, it is possible to locate the light source within the integrating cavity for greater efficiency. Again, the filtration involves elimination of the infra-red light for scanning fully developed film images.
Others have suggested placing discrete lamps in a row within a light integrating cavity, as illustrated in Japanese Patent Application 62-275247, but have noted the problems in intensity variation along the length of the line of light attributed to lamp pitch. In U.S. Pat. No. 4,814,667, a linear light source for a photo copier employing an array of LEDs imaged through a lens is described, wherein the spacing between groups of LEDs is varied to even out the intensity variations encountered with even spacing.